As discussed in U.S. Pat. No. 5,143,096 (Steinberg), a number of methods are known for expanding cellular materials, including tobacco and other agricultural products. In general, these methods involve introducing an expansion agent, i.e., a substance capable of undergoing expansion, as by a phase change from a liquid to a gas, into the cells of the material and causing the agent to expand.
It also is known to expand cellular material by impregnating it with a liquefied gas expansion agent, such as liquefied carbon dioxide, at an elevated pressure; removing excess expanding agent from the cellular material; reducing the pressure in which the cellular material is contained, thereby causing the expansion agent to solidify; and heating the cellular material, such as by exposure to a hot gas stream, e.g., steam, air, etc., to cause the solidified expansion agent to evaporate or sublime. The solidified expansion agent vaporizes at a rate greater than the rate at which the agent in gaseous form can escape from the cellular material. As a result of this treatment, the material is forced to expand.
The use of carbon dioxide as an expansion agent for expanding tobacco also is discussed in U.S. Pat. No. 4,235,250 (Utsch); U.S. Pat. No. 4,258,729 (de la Burde et al.); and U.S. Pat. No. 4,336,814 (Sykes et al.), among others. In the processes disclosed in those patents, carbon dioxide, either in gas or liquid form, is contacted with tobacco for impregnation, and thereafter the impregnated tobacco is subjected to rapid heating conditions to volatilize the carbon dioxide and thereby expand the tobacco.
U.S. Pat. No. 4,340,073 (de la Burde et al.) discloses a process and apparatus for expanding tobacco by impregnating the tobacco with carbon dioxide under conditions such that the carbon dioxide in contact with the tobacco is in liquid form, removing excess liquefied carbon dioxide from the tobacco, reducing the pressure of the impregnated tobacco to solidify carbon dioxide within the tobacco structure, and rapidly heating the tobacco at atmospheric pressure to vaporize the carbon dioxide and expand the tobacco.
U.K. Patent Specification 1,484,536 (Michals) discloses a method for expanding an organic substance, such as tobacco, using liquid carbon dioxide. The method comprises the steps of pressurizing a vessel containing the substance to be expanded to a pressure in the range of about 200 to 1,070 psi with carbon dioxide, immersing the substance in liquid carbon dioxide while maintaining the pressure within the vessel, thereby impregnating the substance with the liquid carbon dioxide, removing excess liquid carbon dioxide from the impregnation vessel, depressurizing the vessel to substantially atmospheric pressure, thereby causing liquefied carbon dioxide on and in the substance to solidify, removing the impregnated substance from the vessel, and heating the substance to cause expansion of the substance by at least 10%. In this method, the carbon dioxide used to pressurize the impregnation vessel is taken from the vapor space of the process vessel that is used to provide liquid carbon dioxide to the impregnation chamber. After removal of the liquid carbon dioxide from the impregnation chamber, the carbon dioxide residue gas in the impregnation chamber is vented to the atmosphere or to a carbon dioxide recovery system (which is not shown in that patent specification).
Various types of recovery systems for carbon dioxide and other expansion agents (e.g., propane) used in tobacco expansion processes are disclosed in the prior art, as discussed below.
U.S. Pat. No. 4,165,618 (Tyree, Jr.) discloses a process for treating products, such as tobacco, using a liquid cryogen, such as liquefied carbon dioxide. In this process, a vessel in which the tobacco is impregnated is purged and pressurized by transferring gas from the vapor space of a liquid cryogen storage vessel to the impregnating vessel. Subsequent to pressurization, liquid cryogen is transferred to the impregnation vessel from the liquid storage vessel. The tobacco is permitted to soak in the liquid cryogen for a predetermined time period, after which it is returned to the liquid storage vessel. The gaseous cryogen remaining in the impregnation vessel after removal of the liquid cryogen is then transferred to a series of accumulators from which the gas is compressed and eventually returned to a main reservoir for the liquid cryogen.
U.S. Pat. No. 5,365,950 (Yoshimoto, et al.) discloses an apparatus for expanding tobacco which uses carbon dioxide as an expansion agent and recycles the carbon dioxide using a pressure swing absorption (PSA) apparatus. The PSA apparatus is used as a recovery/separation unit to separate air (an impurity gas) from the recovered carbon dioxide. The carbon dioxide is then compressed to a higher pressure and supplied to an impregnating vessel. Several alternative embodiments are described which utilize one or more compressors to increase the pressure of the recovered carbon dioxide.
U.S. Pat. No. 5,311,885 (Yoshimoto, et al.) discloses another apparatus for expanding tobacco which uses carbon dioxide as an expansion agent and recycles the carbon dioxide using a PSA apparatus for recovery/separation of the carbon dioxide, similar to that in U.S. Pat. No. 5,365,950.
U.S. Pat. No. 5,711,319 (Cumner) discloses a process for the expansion of tobacco using carbon dioxide. Carbon dioxide gas discharged from an impregnator vessel during the depressurization step is collected within a carbon dioxide recovery balloon. Gas within the recovery balloon is recompressed using a compressor and is reliquified by a heat exchanger before being returned to a process vessel. The carbon dioxide reservoir is recharged with carbon dioxide gas directly from the compressor. Alternatively, carbon dioxide gas discharged from the impregnator vessel during the depressurization step is collected within an intermediate pressure vessel which conserves the pressure of a portion of the vented gas, the remainder being discharged to the recovery balloon. Preferably, a compressor is provided to transfer gas from the recovery balloon to the intermediate pressure vessel and a second compressor is used to transfer gas to a heat exchanger. Reliquified carbon dioxide from the heat exchanger is then returned to the processed vessel. The gas to recharge the reservoir with carbon dioxide is obtained directly from the second compressor.
U.S. Pat. No. 5,819,754 (Conrad, et al.) discloses an apparatus and processes for expanded tobacco with an expansion agent, such as propane. Following a pre-determined impregnation period, some of the expansion agent is released from the impregnation zone to an accumulator for recycling. (The propane that is recycled back to the accumulator is used in subsequent tobacco treatment cycles.) An expansion agent recovery line is provided to further remove propane that remains in the impregnation zone and is not recycled due to equalization of pressures in the accumulator and chamber. It also provides for periodic removal of high-pressure expansion agent from the impregnation zone so that contaminants (e.g., moisture, etc.) do not build up to undesirable levels in the expansion agent. The expansion agent recovery line is connected to an optional gas recovery or disposable zone (not shown in the patent) for recovery of expansion agent or recovery of energy therefrom.
The tobacco expanding apparatuses may be classified generally into batch-type expanding apparatuses and continuous-type expanding apparatuses. In a typical batch-type expanding apparatus, a predetermined amount of tobacco material is stored in an impregnating vessel, high-pressure carbon dioxide is supplied to the impregnating vessel to impregnate the tobacco material with carbon dioxide, and thereafter the tobacco material is removed, thereby expanding the tobacco material. In a continuous-type expanding apparatus, the tobacco material and carbon dioxide are continuously supplied to an impregnating vessel.
Although the batch-type apparatus has a simple structure, its efficiency is low and a large amount of carbon dioxide is lost. The latter continuous-type expanding apparatus supposedly is more efficient and can recover and reuse carbon dioxide, as indicated in the discussion above for the patents issued to Yoshimoto, et al.--U.S. Pat. Nos. 5,311,885 and 5,365,950.
Many of the conventional processes, including the dry ice expanded tobacco (DIET) process and other carbon dioxide expansion processes, do not recover and reuse all of the available expansion agent (e.g., carbon dioxide), some of which is vented to the atmosphere. In addition to increased emissions to the environment, this results in less than ideal performance of the processes in terms of efficiency and economics.
It is desired to have an improved process and system for the expansion of agricultural products, such as tobacco, food, or other such materials, which overcome the disadvantages of the prior art.
It is further desired to have a more efficient and economic process and system for the expansion of agricultural products, such as tobacco, food, or other such materials.
It is still further desired to have an improved process and system for the expansion of agricultural products, such as tobacco, food, or other such materials which use carbon dioxide as the expansion agent.
It is still further desired to have an improved process and system for expanding agricultural products such as tobacco, food, or other such materials having an improved method and an apparatus for recovering additional amounts of carbon dioxide or another such expansion agent in such process or system.
It is still further desired to have an improved process and system for expanding agricultural products, such as tobacco, food, or other such materials having improved hydrate formation means which results in better expansion of the product and more uniformity of the expansion.